The present invention relates to an original-document illumination apparatus used for digital copiers and image scanners, and also relates to an image reading apparatus, a color-original-document reading apparatus, and an image forming apparatus, in each of which the original-document illumination apparatus is used.
Development of Light Emitting Diodes (LEDs) has been advanced recently, and LEDs are getting more and more luminous dramatically. LEDs, in general terms, have such advantages as longer service life, higher efficiency, higher resistance to impact, and their capability of emitting monochromatic lights. For this reason, application of LEDs to a variety of lightening purposes is expected. Among other purposes, LEDs are actually used in an original-document illumination apparatus included in an image reading apparatus such as a digital copier and an image scanner.
Despite the above-mentioned excellent properties, a single LED cannot emit light that is luminous enough to be used in an illumination apparatus of an image reading apparatus. Accordingly, LEDs are used mainly for such apparatuses as a lower-speed reading apparatus and an apparatus with more emphasis on its compactness. Meanwhile, mainly used for higher-speed reading apparatuses and in larger apparatuses are cold cathode fluorescent lamps.
The above-mentioned problem of LEDs is generally dealt with by use of many LEDs arranged into an array to obtain increased amount of light from the LED array as a whole. In this case, however, the light spreads so widely as to lessen the efficiency and to contradict the promotion of energy saving products. Use of round lamp type LEDs, which suffers less from the spread of their light, may possibly enhance efficiency, but the light emitted from an LED of that type is irradiated in a direction at a narrower angle, so that an uneven luminance distribution occurs in the main-scanning direction.
In an already-proposed original-document illumination apparatus, such as inventions disclosed in Japanese Patent Application Laid-open Publications Nos. H11-232912 and H08-111545, an LED array and a lengthy lens are combined to accomplish more efficient use of light. The improvement in efficiency is generally pursued by making the light from LEDs converge on a sub-scanning cross section of the LEDs. Such a method, however, has a problem. As a drawing of Japanese Patent Application Laid-open Publication No. H11-232912 shows, the center portion of the convergent light is bright, but a rapid drop in luminance is observed at a position away from the center. In this method, among the light emitted from the LEDs, most of the light emitted at an oblique angle to the sub-scanning cross section is wasted, so that an uneven luminance distribution occurs in the main-scanning direction unless many LEDs are arranged.
Incidentally, some original-document illumination apparatuses employ cylindrical lenses. Also in this case, an uneven luminance distribution occurs in the main-scanning direction. FIGS. 1 and 2 describe how an uneven luminance distribution in the main-scanning direction occurs in an original-document illumination apparatus equipped with a cylindrical lens 4 and an LED 3. FIG. 1 illustrates a vertical cross section, and FIG. 2 illustrates a horizontal cross section. Light beams illustrated as solid lines in FIGS. 1 and 2 represent divergent light beams each of which is emitted from the LED 3 and each of which enters the cylindrical lens 4 in a single sub-scanning cross section including the LED 3. In other words, these solid lines represent perpendicular incident light 1a. The dotted lines in FIGS. 1 and 2 represent divergent light beams emitted in a cross-sectional direction at an oblique angle θ illustrated in FIG. 2 with the sub-scanning cross section including the LED 3. In other words, these dotted lines represent oblique incident light 1b. 
FIG. 1 illustrates that the arrangement of the LED 3 relative to the cylindrical lens 4 is not changed between the case of the perpendicular incident light 1a and the case of the oblique incident light 1b. After passing through the cylindrical lens 4, the perpendicular incident light 1a, represented by the solid lines, enters a contact glass 5 as a parallel light. Meanwhile, the oblique incident light 1b, represented by the dotted lines, converge on the contact glass 5, after passing through the cylindrical lens 4.
This is because, as FIG. 2 illustrates, the perpendicular incident light 1a of the solid lines and the oblique incident light 1b of the dotted lines pass through the cylindrical lens 4 via different routes. The difference in route derives from the following face. The oblique incident light 1b of the dotted lines passes diagonally across the cylindrical lens 4, so that the light beam of the oblique incident light 1b has a large apparent curvature of the light incident plane and that of the light output plane (i.e., the curvature radius of each planes becomes shorter).
Now, assume that the LED 3 has a light distribution (angular distribution of the intensity of light irradiation) of, for example as FIG. 3 illustrates, a distribution according to Beer-Lambert Law (as the angle Φ becomes larger, the light intensity of the LED 3 is decreased from the largest light intensity in the front direction in proportion to the value of cos Φ). In this case, when only this single LED 3 and the cylindrical lens 4 are used to illuminate the contact glass 5, the illuminance distribution in a luminance measurement area 6 illustrated in FIG. 2 in the main-scanning direction X has peaks as FIG. 4 represents.
To counter the uneven luminance distribution in the main-scanning direction, the present applicant proposed an invention disclosed in Japanese Patent Application Laid-open Publication No. H10-322521, but there is no mention as to how to condense light in the sub-scanning direction. In Addition, the present applicant proposed another invention, as disclosed in Japanese Patent Application Laid-open Publication No. 2004-361425 with a configuration having a light guiding member, as an optical element, that has its light incident plane arranged near the light output plane of the light beams emitted from a point light source, and its light output plane faced towards the reading area. According to this configuration, the favorable, targeted distribution of luminance is obtained. The apparatus, however, needs a rather complex structure as it uses reflector plates in addition to the light guiding member. This requires a higher cost.
The present applicant proposed a still another invention as disclosed in Japanese Patent Application Laid-open Publication No. 2005-278132. An original-document illumination apparatus disclosed therein is equipped with a light-source unit in which LEDs with a certain distribution of luminous intensity are installed as arranged in a plurality of rows, and is also equipped with a lengthy lens that does not make the light passing therethrough converge in the direction of the sub-scanning cross-section. The light-source unit and the lengthy lens are appropriately arranged so that the light emitted from each of the LEDs is made to converge not at a position on the area of the original-document on the corresponding sub-scanning cross section to the same LED. Rather, the light is made to converge at another position on the area of the original-document, and the position is shifted in the main-scanning direction so that an angle is made with the sub-scanning cross section. The original-document illumination apparatus thus obtained has a high convergence of light in the sub-scanning direction, allows a larger numerical aperture (NA), and suffers from a less loss due to the spreading of light in the main-scanning direction. In sum, the proposed original-document illumination apparatus accomplishes less unevenness in luminance distribution with a relatively small number of LEDs.
The invention disclosed in Japanese Patent Application Laid-open Publication No. 2005-278132 has advantages mentioned above, but also has some drawbacks. For example, the acute manner of converging the light at a position on the area of the original-document, when combined with a misalignment of illuminating position caused, for example, by an incorrect angle with which the lengthy lens is attached, may cause a big change in light quantity that reaches a light detector of a line sensor. This may possibly affect the image to be formed by the apparatus. For this reason, it is preferable that, in the original-document illumination apparatus used in image forming apparatuses such as a digital copier and an image scanner, the illumination-intensity-distribution curve in the sub-scanning direction be wide to some extent. The curve is preferable because no difference in luminous intensity within the reading area is caused even when the center position of illumination is displaced from the reading part. To this end, there may preferably be a flat section in the intensity curve near the section with the maximum value in the distribution of luminous intensity. In the flat section, less unevenness in luminous intensity occurs over the width larger than the width formed by adding the width needed for the reading and the margin for accommodating the mechanical error and the like. For example, 1-mm flat portion on each side is preferable.
For the foregoing reasons, there is a need for an original-document illumination apparatus, an image reading apparatus, a color-original-document reading apparatus, and an image forming apparatus that each has a flat portion of a necessary width near the maximum value in the luminous-intensity-distribution curve, so that the apparatus does not suffer from the influence of the mechanical error or causes no difference in luminous intensity within the reading area, and, in addition, the apparatus is simple in its structure, and high in light-use efficiency.